Method for fabricating P-wells and N-wells having optimized field and active regions

Abstract

A method forms, in a CMOS semiconductor substrate, P- and N-wells having independently optimized field regions and active regions. In one embodiment, P- and N-wells are formed by (i) creating in successive steps the field regions of the P- and N-wells; (ii) creating an oxide layer over the field regions, (iii) creating in successive steps the active regions. The method achieves the P- and N-wells without increasing the number of photoresist masking steps. In addition, optical alignment targets (OATs) are optionally formed simultaneously with these P- and N-wells without increasing the total number of process steps.

We claim: 
     
       1. A method for fabricating on a semiconductor substrate a first well of a first conductivity and a second well of a second conductivity opposite said first conductivity, said method comprising: forming a first field region outside a region of said semiconductor substrate designated for said first well, said step of forming said first field region comprises the steps of: forming an oxide layer over the entire surface of said semiconductor substrate; forming a nitride layer over said oxide layer;   patterning said nitride layer using a first photoresist mask to expose portions of said nitride layer above said first field region;   removing said exposed portions of said nitride layer; and   implanting a dopant of said first conductivity type into said first field region;     forming a second field region outside a region of said semiconductor substrate designated for said second well;   forming a protective mask layer above said first and second field regions;   forming a first active region within said first well; and   forming a second active region within said second well.   
     
     
       2. A method for fabricating on a semiconductor substrate a first well of a first conductivity and a second well of a second conductivity opposite said first conductivity, said method comprising: forming a first field region outside a region of said semiconductor substrate designated for said first well;   forming a second field region outside a region of said semiconductor substrate designated for said second well;   forming a protective mask layer above said first and second field regions, said protective mask layer comprising a LOCOS oxide layer;   forming a first active region within said first well; and   forming a second active region within said second well.   
     
     
       3. A method as in claim 1, wherein said first and second wells are formed by dopant diffusion in one or more thermal cycles in process steps subsequent to said implanting step. 
     
     
       4. A method as in claim 1, wherein said step for forming said first field region also forms a region designated for an optical alignment target, using a mask extension covering a designated area for said optical alignment target. 
     
     
       5. A method as in claim 4, wherein said region designated for said optical alignment target is formed by the steps of: removing, subsequent to said step of removing said exposed portions of said nitride layer, a portion of said semiconductor substrate underneath said exposed portions of said nitride layer to a predetermined depth, thereby exposing portions of said semiconductor substrate; and   providing a layer of oxide over said exposed portions of said semiconductor substrate.   
     
     
       6. A method as in claim 5, wherein said layer of oxide over said exposed portions of said semiconductor substrate is formed by providing a LOCOS oxide. 
     
     
       7. A method as in claim 4, wherein said region designated for said optical alignment target is formed by the steps of: forming, subsequent to said step of removing said exposed portions of said nitride layer, a layer of LOCOS oxide in place of said exposed portions of said nitride oxide; and   removing from said LOCOS oxide a predetermined depth of said LOCOS oxide.   
     
     
       8. A method as in claim 4, wherein said region designated for said optical alignment target is formed by the steps of: forming, subsequent to said step of removing said exposed portions of said nitride layer, a layer of LOCOS oxide in place of said exposed portions of said nitride oxide;   removing completely said LOCOS oxide; and   forming a predetermined thickness of a second oxide layer in place of said removed LOCOS oxide.